The invention relates generally to the field of power over local area networks, particularly Ethernet based networks, and more particularly to a redundant powering system of a device powered over communication cabling.
The growth of local and wide area networks based on Ethernet technology has been an important driver for cabling offices and homes with structured cabling systems having multiple twisted wire pairs. The ubiquitous local area network, and the equipment which operates thereon, has led to a situation where there is often a need to attach a network operated device for which power is to be advantageously supplied by the network over the network wiring. Supplying power over the network wiring has many advantages including, but not limited to: reduced cost of installation; centralized power and power back-up; and centralized security and management.
The IEEE 802.3af-2003 standard, whose contents are incorporated herein by reference, is addressed to powering remote devices over an Ethernet based network. Power, with a maximum of 12.95 watts, can be delivered to the powered device (PD) either directly from the switch/hub known as an endpoint power sourcing equipment (PSE) or alternatively via a midspan PSE. The term PD comprises a LAN node receiving power over the communication equipment. Unfortunately, no provision has been made in the above standard for PDs requiring power in excess of the above maximum power requirement. Furthermore, no provision has been made in the above standard for redundancy, i.e. a secondary PSE and path for powering the PD upon failure of the primary PSE. A redundant powering system would enhance reliability ensuring near constant operation, preferably to a level of 99.999% (Five 9's reliability). One important market for powering devices over Ethernet cabling is IP telephony, which is typically utilized to replace highly reliable enterprise wide telephone equipment. Often the enterprise wide telephone equipment to be replaced exhibits five 9's reliability, and as such, ensuring reliability of an enterprise IP telephone network is a critical feature.
The above mentioned standard further requires that a PD be capable of accepting power on either of two powering paths: a first path comprising conductors 1, 2, 3 and 6; and a second path comprising conductors 4, 5, 7 and 8. The PD is to be implemented so as to be insensitive to the polarity of the power supply. In an exemplary embodiment a PD according to the above standard exhibits a diode bridge at the input thus implementing this requirement.
In a typical application, a PD comprises a PD interface circuitry and a PD operational circuitry. The PD interface circuitry enables detection and optional classification in accordance with the above standard. Power is isolated by the PD interface circuitry from the PD operational circuitry through an isolating switch, and is enabled to the PD operational circuitry only after the voltage at the PD, supplied from the PSE via the communication cabling, rises to Von. One function of the PD interface circuitry is thus to close the isolating switch thereby enabling operation of the PD operational circuitry. In a typical application, the output of the isolating switch is fed to the input of a DC/DC converter, and the output of the DC/DC converter powers the PD operational circuitry.
The standard further prescribes a power turn on time, designated tpon. In the event that the PSE powers the PD, power is to be supplied within tpon of a valid detection. Once steady state powering has begun a disconnect detection function is to be active. The PSE may monitor an AC maintain power signature (MPS) component only, a DC MPS component only or both the AC and the DC MPS components. Power is to be removed from a PD in the absence of a valid MPS.
U.S. patent application Ser. No. 10/761,327 filed Jan. 22, 2004 entitled “High Power Architecture for Power Over Ethernet”, whose contents are incorporated herein by reference, is addressed to the issue of supplying power in excess of the above mentioned limit of 12.95 watts per PD.
It would therefore be desirable to have a redundant powering system for powering a PD over communication cabling.